Some cleanup in package.xml and CMakeLists.txt

 - builds broke sporadically (I think because of the missing *_gencpp in
   add_dependencies) with missing Packet.h file.
 - I’m no catkin expert, but these changes make catkin_lint happy (no
   more errors at least).
Merge pull request #13 from NikolausDemmel/patch1

Some cleanup in package.xml and CMakeLists.txt